The present disclosure relates generally to a nuclear magnetic resonance (NMR) method and apparatus for evaluating a characteristic of a region, and particularly for evaluating a characteristic of a fluid in a subterranean region.
NMR oil well logging is a technique used to investigate subterranean regions that may contain oil reserves. The nuclei of chemical elements have a characteristic angular momentum (spin) and a magnetic moment, and by detecting and analyzing the reaction of the nuclei to applied magnetic fields, the characteristics of specific nuclei may be deduced. In the presence of an externally applied static magnetic field (B0), the nuclei spins become magnetized and align themselves parallel to the B0 field. By applying a radio frequency (RF) pulse train of a specific frequency to the magnetized nuclei, a pulsed RF magnetic field (B1) is generated that tips, or flips, the spins away from the direction of the B0 field. If the RF frequency (ω) matches the condition for NMR (ω=γB0), where γ is the gyromagnetic ratio, then the first pulse reorients the magnetization to start precession and subsequent pulses generate spin-echo signals. A RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence is typically used for oil well logging.
At the end of a ninety degree tipping pulse, the spins are oriented transverse to the B0 field and precess around the direction of the B0 field at the Larmor frequency (ω0=γB0), and the transverse magnetization dephases with a transverse relaxation time constant (T2), also known as the spin-spin relaxation time. Repeated tipping and relaxation of the spins results in the NMR spin-echo signal, which may then be detected and analyzed for oilfield exploration. However, heavy oil is difficult to discriminate from bound water using conventional NMR well logging contrasting techniques because their relaxation times are similar. Even though the diffusion constant of heavy oils is at least ten times smaller than that of water, changes in T2 are small for typical logging echo times. Accordingly, there is a need in the art for a NMR detection and analysis method that overcomes these drawbacks.